Hello,
This is a widespread issue. It is also discussed in the Microsoft Q&A post: Upgrading to Windows 11 24H2 causes remote desktop connections to Windows 7 machines not to show graphics correctly. - Microsoft Q&A
A temporary solution involves replacing mstsc.exe and mstscax.dll file with one from an earlier Windows version, such as from 23H2.
I regret that I cannot provide the exact root cause of this issue at this time. However, I believe the development team is already investigating it and working diligently to find a solution.
You can also submit feedback to Microsoft through the Feedback Hub app: Send feedback to Microsoft with the Feedback Hub app - Microsoft Support
I hope this information helps.
Best regards,
Karlie Weng